Wood Rot vs. Mold: How to Tell Them Apart

Mold is a surface growth: it discolors wood, can often be brushed or planed off softwoods, and on its own reduces strength only slightly. Rot, also called decay, is fungi digesting the wood itself, and it can cut strength by more than half before anyone can see it. Both need moisture, so the repair always starts with finding the water.

The source for most of what follows is the Wood Handbook, published by the USDA Forest Service’s Forest Products Laboratory, the federal research lab for wood. Its chapter on biodeterioration appears in the 2021 edition, publication FPL-GTR-282.

What mold on wood looks like

Mold shows up as fuzzy or powdery growth on the surface. The Forest Products Laboratory describes colors ranging from light shades to black, with green and yellowish tones common among the brighter ones.

How deep it goes depends on the wood. On softwoods such as pine, the discoloring surface growth often can be brushed or planed off, even though the fungus may reach deeper. On large-pored hardwoods such as oak, the stain beneath the growth usually runs too deep to surface off.

Mold’s effect on strength is small. According to the laboratory, stain and mold fungi affect the strength of wood only slightly; their main effects are on appearance and on toughness, the ability to absorb a sudden impact. Mold does have two side effects worth knowing. It makes wood more absorbent, which lets it take on water more easily and invites decay fungi. And early decay may already be present, unnoticed, in the discolored areas.

What rot looks like

Decay changes the wood itself. On the surface, decay fungi can appear as fan-shaped patches, strands or root-like structures, usually white or brown and sometimes bright yellow or orange. Some produce fruiting bodies shaped like mushrooms, brackets or crusts.

The laboratory describes two main kinds:

Brown rot removes cellulose. The wood turns browner, cracks across the grain into cube-like pieces, shrinks, collapses and can be crushed into powder.
White rot removes both lignin and cellulose. The wood may look bleached, does not crack across the grain, keeps its outside dimensions until the decay is severe, and often feels spongy.

A third kind, soft rot, stays shallow and mostly affects the outer surface. It is soft when wet, with firm wood just beneath.

Early decay is hard to spot. The handbook notes that its first stages bring discoloration that can be difficult to recognize, such as abnormal mottling with unnaturally brown or bleached areas.

“Dry rot” is a misleading name

There is no rot without water. The Forest Products Laboratory says brown, crumbly rot is sometimes called dry rot when it is found dry, but the term is incorrect, because wood must have available moisture to decay, even if it dries out later.

One group of brown-rot fungi explains the old name. These fungi can form water-conducting strands that carry water, usually from the soil, into a building and wet wood that would otherwise stay dry. The laboratory prefers the term water-conducting fungi, because like all decay fungi they still need water.

How much strength rot takes

A great deal, and early. In laboratory tests cited in the handbook, wood lost from 6 percent to more than 50 percent of its toughness by the time decay had consumed just 1 percent of its weight. By 10 percent weight loss, most strength losses may exceed 50 percent, and at that stage decay may be detectable only under a microscope.

The handbook’s conclusion: wood with decay you can see has been greatly reduced in all strength values. Brown rot causes larger strength losses in its early stages than white rot.

For a floor joist, a sill plate or a rafter, that is the difference between a cosmetic problem and a structural one.

The moisture numbers that matter

About 20 percent moisture content is the safe side of the line, and about 30 percent is the level above which serious decay occurs. The handbook’s thresholds:

Serious decay occurs only when the wood’s moisture content is above the fiber saturation point, about 30 percent on average.
Fully air-dried wood usually has a moisture content no higher than 20 percent, which provides a reasonable margin of safety against fungal damage.
Wood kept air-dry will not decay, and decay already present will not progress.
Humid air alone does not wet wood enough for significant decay, but it does allow some mold to develop.

Temperature matters too. Most decay fungi have their optimum growth around 77°F, slow down below 50°F and above 95°F, and essentially stop near 35°F or 100°F.

A moisture meter reading, taken by you or a contractor, turns those thresholds into something you can check. Moldy wood that stays at 20 percent or below is within the handbook’s margin of safety against decay, and the mold on it is a cleaning job. Wood that reads well above that, or that is soft, cracked into cubes or spongy, needs a closer look.

Where rot starts in a house

Wherever water stays: leaks, crawl spaces, soil contact and roof edges. The handbook says serious decay in buildings is almost always a sign of faulty design or construction, careless handling of the wood, or poor maintenance. The usual trouble spots:

plumbing leaks, particularly ones that go unnoticed for a long time,
crawl spaces, where condensation from soil moisture can wet floor framing,
wood in contact with soil; the laboratory recommends at least 8 inches between soil and framing and 6 inches between soil and siding,
roof edges and places where roofs meet walls.

It also recommends a roof overhang of about 2 feet, with gutters and downspouts kept clear of debris. The National Park Service’s list of warning signs includes cupped, warped, cracked or rotted wood alongside wet stains and musty smells.

What about termites?

Termites also damage wood, but they call for a pest control professional, not a mold or water contractor. The handbook calls termites the major insect pest of wood, and it notes that on a national scale they are a less serious threat than fungi.

When to call a professional, and what to ask

Call a carpenter or a structural engineer when wood is soft, crumbling, cracked into cubes or spongy, or when the affected piece carries weight: joists, beams, sill plates, rim joists, rafters or the subfloor around a toilet or tub. The moisture source may need a plumber or roofer. If mold covers more than about 10 square feet, the EPA’s threshold for doing the cleanup yourself, add a mold remediation contractor.

The handbook’s repair principles are simple. Correct the moisture condition that caused the decay. If the moisture source is fully eliminated, it is enough to replace the weakened wood with dry lumber. If it cannot be corrected, use preservative-treated wood or the heartwood of a decay-resistant species.

Questions worth asking:

Where on the damaged piece were moisture readings taken, and how do they compare with the handbook’s 20 and 30 percent marks?
Is this surface mold, or decay? How far does the decay extend, and how did you determine that?
What caused the wetting, and who is fixing it?
Will structural members be replaced or reinforced, and does that need a permit?
If the area cannot be kept dry, will the replacement wood be preservative-treated or naturally decay-resistant?

For mold on sound wood, the EPA’s basic method applies: fix the water problem first, then “scrub mold off hard surfaces with detergent and water, and dry completely.”